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05 Nov 2004
Researchers from Spain and Mexico use a fiber laser's buildup time to measure hydrogen concentration.
An optical hydrogen sensor based on an erbium-doped fiber laser could make fuel cells safer to use and reduce the cost of industrial and environmental monitoring. Researchers from Universidad de Valencia, Spain, and Centro de Investigaciones en Óptica, Mexico, have used their fiber set-up to monitor an increase in hydrogen levels from 0 to 10% with a resolution of better than 0.1%. (Optics Letters 29 2461)
At the heart of the detector is a palladium (Pd) coated tapered fiber which is inserted into the cavity of an erbium-doped fiber laser. "An important aspect of Pd is the fact that its reaction with hydrogen is almost completely reversible at room temperature," researcher Arturo Ortigosa-Blanch told Optics.org.
Exposure to hydrogen changes the tapered fiber's attenuation which modifies the dynamics of the laser. This means that the team can determine the concentration of hydrogen in the surrounding air simply by measuring the buildup time of the laser - the time delay between the pump being switched on and the appearance of the first laser pulse.
Because the sensor relies on timing data, Ortigosa-Blanch feels that it has an advantage over competing technology. "Translating the measurement into the time domain makes it possible to acquire and process the information very easily and accurately using reliable and low-cost electronics," he explained. "Also, fibre-optic based sensors are immune to electromagnetic interference and are safer in flammable environments where a spark could cause a detonation."
A key breakthough in the sensor's development was the discovery that biasing the laser with a certain pump power (below the laser threshold) gave a dramatic boost in sensitivity. The researchers found that for a hydrogen concentration of 10%, the relative buildup time increased from 0.1 to almost 0.6 when the bias pump power and high-level pump power settings were raised from 3 mW and 84 mw to 8 mW and 93 mW respectively.
Although still focusing on hydrogen because of its industrial importance, the group has longer term plans to orientate its work towards biological sensors.
Author
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.
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